The invention relates to a device for testing ferromagnetic materials, such as pipelines and the like, for faults, cracks, corrosion, etc., using at least one electromagnetic ultrasonic transducer with at least one high frequency current coil in connection with a magnetic field for exciting and/or detecting ultrasonic waves in the material wall, and at least one magnet system producing the magnetic field.
Non-destructive material testing can take place employing ultrasonics, where piezoelectric transducers are used, from which are emitted ultrasonic waves and by means of a coupling medium, such as e.g. water, the waves; are introduced into the workpiece. For the ultrasonic testing of gas pipelines for e.g. crack-like faults on the outside of the pipe, it is only possible to have a dry coupling to the inside of the pipe, e.g. employing airborne noise, ultrasonic excitation by means of a laser, or electromagnetic ultrasonic excitation.
DE-OS 35 11 076 e.g. discloses a test scraper or go-devil for the non-destructive testing and monitoring of pipeline walls made from ferromagnetic material, in which for the detection of wall weaknesses due to corrosion from the outside or inside, a magnetic field, produced there by one of the stray field measuring systems, is used for the electrodynamic excitation of ultrasonics and stray flux measurement. A scraper is provided with uniformly circumferentially distributed electromagnets, which in each case have two aligned measuring heads, a yoke connecting said measuring heads and a magnetic coil on said measuring heads. The field of each electromagnet is parallel to the pipe centre axis. For ultrasonic measurement purposes an air coil is placed directly on at least one of the poles or measuring heads, which is subject to the action of strong and very steep-sided current pulses. The running time of the ultrasonic waves produced by these current pulses, together with the magnetic field in the material acting in the material wall area, is determined and evaluated. It is disadvantageous when using such a testing scraper that Barkhausen noise occurs on moving the measuring head provided with the high frequency coil. This noise is produced in that the ferromagnetic domains in the ferromagnetic material during the movement suddenly reverse to a new equilibrium position as a result of a strong change in the magnetic field. Consequently a current is induced, which can be heard as clicking with an amplifier and a loudspeaker. In the case of rapidly succeeding field changes there is a crackling noise. Therefore the signal-to-noise ratio is inadequate to allow a reliable measurement to take place.
In other known electromagnetic ultrasonic transducers used for the ultrasonic testing and material characterization of electrically conductive materials, e.g. those described in EP-A-609 754, there are at least two rows of permanent magnets with alternating pole distribution and a HF coil positioned below the same for exciting horizontally polarized transverse waves (SH-waves). Also with these ultrasonic transducers, in the dynamic testing of ferromagnetic materials there is an increased background noise, i.e. a reduction of the usable dynamic range, because during the movement of the electromagnetic ultrasonic transducer over a ferromagnetic component Barkhausen noise occurs due to the continuous turn around/magnetic reversal of the magnetic moments within the individual domains of the material. The level of this noise, as a function of the scanning rate, is up to 25 dB.